In the typical long term evolution (LTE), bearer management for interworking between an LTE base station and an evolved packet core (EPC), which is a core network, is supported through an S1 interface and application protocols.
Meanwhile, with a 5G network introduced newly, it is essentially needed to provide mobility between 5G base stations. Especially, in case a 5G base station uses an mmWave frequency of a high-frequency band (e.g., 28 GHz), it is expected that the coverage of the base station will be further reduced due to the characteristics of the frequency. In the 5G network, a user equipment may move more frequently between base stations. Therefore it is very important to provide a handover procedure for addressing such an issue.
In addition, in case a base station employing a 5G radio access technology is deployed in a coverage area where a base station employing a typical LTE radio access technology is located, there is a distinct possibility that a handover between the LTE base station and the 5G base station may occur frequently.
In such a situation, unlike transmission on a bearer basis between an LTE core network and an LTE base station, in the 5G, it is possible to control quality on a quality of service (QoS) flow basis. It is also possible to transmit packet data between a 5G core network (hereinafter, referred to as 5G core, 5G core network, NGC or 5GC) and a 5G base station (hereinafter, referred to as 5G NB, NR NB, NG-RAN or gNB) through a packet data unit (PDU) session containing the QoS flow.
Thus, it is necessary to enable a user equipment to perform a handover in a heterogeneous network environment where different radio access technologies are employed and different core systems are installed. It is also necessary to develop technologies for efficiently controlling a handover in the heterogeneous network environment according to characteristics of the different networks.